1. Field of the Invention
The present invention relates to an organic light emitting display to improve a contrast ratio and prevent roll-off at a low gray scale.
2. Discussion of the Related Art
The recent advent of information-dependent age has brought about rapid development in the display field that visually displays electrical information signals. In this regard, a variety of flat panel displays having superior properties such as slimness, light weight and low power consumption have been developed.
Examples of flat panel displays include liquid crystal displays (LCDs), plasma display panels (PDPs), field emission displays (FEDs), organic light emitting displays (OLEDs), and the like.
In particular, an organic light emitting display spontaneously emits light, which has advantages of rapid response speed, superior luminous efficacy, excellent brightness and wide viewing angle, as compared to other flat panel displays. Such an organic light emitting display includes an anode and a cathode which face each other such that a light emitting layer is interposed therebetween. Holes injected from the anode recombine with electrons injected from the cathode in a light emitting layer to form electron-hole pairs, i.e., excitons. When the excitons transition to a ground state, energy is released. Based on this energy, the organic light emitting display emits light.
However, conventional organic light emitting displays have a disadvantage of deterioration in contrast ratio caused by leaked current flowing in light emitting cells during a non-display term of the light emitting cells.
Specifically, when a red light emitting layer of red light emitting cells is formed of a phosphorescent material, the red light emitting layer easily emits light due to considerably high quantum efficiency in spite of low leakage current at a zero gray scale rendering black. In this case, although only the red light emitting cell emits light, the same effects as when green and blue light emitting cells disposed at both sides of the red light emitting cell emit light at a zero gray scale are obtained. As a result, the green and blue light emitting cells as well as the red light emitting cell have disadvantages of deterioration in contrast ratio.
In addition, when the red light emitting layer is formed of a phosphorescent material, the red light emitting layer has the highest efficiency, as compared to green and blue light emitting layers, thus disadvantageously causing a roll-off phenomenon. That is, as shown in FIG. 1, a roll-off phenomenon in which efficiency of the red light emitting layer gradually decreases from a low current to a high current occurs. In particular, an efficiency is high and suddenly decreases at a low gray scale. Accordingly, in a conventional organic light emitting display, at a low gray scale, it is disadvantageously difficult to form a gamma curve showing variation in efficiency of the red light emitting layer with respect to a gray scale.